In the known devices, the hooking means present various shapes, which are a function of the configuration of the legs. In documents FR.A.2 570 288 and U.S. Pat. No. A.3 952 747, they are constituted by the sharp end of each leg which is bent to form a hook. In U.S. Pat. No. A.3 334 629, it is question of a plurality of small teeth projecting from the outer surface of the free end of each leg.
The purpose of these hooking means is to maintain the device at the place where it was implanted in the vessel, avoiding any displacement and any migration of the device, and also avoiding perforation of or injury to the wall of the vessel.
According to Applicant, this purpose is only imperfectly attained in the prior art devices. He has observed that, being designed to oppose any displacement of the device in the direction of blood flow within the vessel, the hooking means mentioned above are not really in a position to ensure definitive and absolute anchoring. In fact, the wall of the vessel is not a rigid element; it is subjected to fluctuating movements during passage of the flow, in particular of the blood flow in a vein. These fluctuating movements act on the legs of the device and may cause unhooking of all or part of these legs of which the hooks or teeth all have the same orientation, with, correlatively, a displacement of the whole device or a relative displacement of one or more legs with respect to the others and therefore a poor positioning of the device with respect to the vessel. These fluctuating movements may, in combination with the driving force of the flux, also cause perforation of the wall when the hooking means are too sharp.